This invention relates to a waste tank for a vacuum sewage system installed in a transport vehicle such as an aircraft, boat, bus or train, particularly a high-speed train.
A vacuum sewage system installed in a transport vehicle includes a waste tank for collecting and temporarily storing sewage. The tank has an inlet opening for connection through a sewer pipe to a source of sewage, such as a toilet, and admitting air and sewage to the tank, and an outlet opening for exhausting air from the tank. The outlet opening is connected through a duct to the suction side of a vacuum generator, for instance a blower, the pressure side of which vents to the atmosphere. Operation of the vacuum generator may be controlled by a pressure sensor, which is located in the duct immediately upstream of the vacuum generator, in the tank or in the sewer pipe, and normally functions to maintain a pressure difference between the tank and the interior of the vehicle above a threshold level, or to produce such a pressure difference when flushing of a toilet, or another sewage-supplying operation, is initiated. In the case of a vacuum sewage system installed in an aircraft, the vacuum generator is not needed when the pressure outside the aircraft cabin is sufficiently low to create the desired pressure difference.
In operation of such as vacuum sewage system connected to a toilet, flushing of the toilet is initiated by actuating a flush button. The pressure sensor disables the flush button from initiating a flushing cycle if the pressure difference sensed by the pressure sensor is below the threshold value. The tank is also provided with a level sensor for disabling the flush button when the tank is full. When a flushing cycle is initiated, a discharge valve connected between the toilet bowl and the sewer pipe is opened. The pressure difference between the interior of the toilet bowl and the interior of the sewer pipe causes sewage in the toilet bowl to be propelled from the bowl into the ser pipe. Air enters the sewer pipe behind the sewage and pushes it towards the tank. The discharge valve is held open for a predetermined time and then closes. After the discharge valve is closed, the sewage in the sewer pipe continues to travel toward the tank as the air upstream of the sewage expands. The flushing cycle is then complete and a new flushing cycle may take place. Preferably, it takes only one flushing cycle to transport sewage from the toilet to the waste tank. When the sewage enters the waste tank, it does so at quite a high speed. Accordingly hard objects, such as small bottles, may impinge on the tank wall with a substantial impact and there is a danger that the tank will be damaged, especially in the case of a vacuum sewage system installed aboard an aircraft since, in order to minimize weight, the waste tank may be of relatively flimsy construction.
It has been proposed that the inlet opening for the waste tank of an aircraft vacuum sewage system be formed in the horizontal top wall of the holding tank and be coupled to the sewer pipe by a connection fitting that makes the sewage enter the tank in a direction vertically downwards. In accordance with this proposal, the outlet opening is spaced from the inlet opening and is provided with a separator for preventing water form leaving the tank.
The proposed aircraft vacuum sewage system is subject to disadvantage, in that forces caused by aircraft movement may result in quite vigorous movement of the sewage in the tank, and this may cause sewage to enter the separator and block it. The pressure drop across the separator would increase, which may result in too high a pressure in the tank and in the sewer pipe. If the toilet was then flushed, there might not be sufficient vacuum in the waste tank for the sewage in the toilet bowl to be propelled completely into the sewer pipe Consequently, there would be an increased probability that the discharge valve would be blocked. Moreover, sewage that has entered the separator might be pushed through the separator into the blower, which might result in damage to the blower. In order to avoid blockage of the separator, the level sensor should be placed so as to disable the flush button when the level of sewage in the tank reaches about 25 cm below the top of the tank. Typically, the height of the tank would be only about 75 cm, and therefore a large proportion of the volume of the tank would not be available for storage of sewage.